Remote air compressor

[mrcaptainbob]

Would like to be able to turn off the remote air compressor. It will be switched over to 240V when it gets shoved up to the barn loft. Although it has it's own on/off cycle of coarse, but how can I shutoff the electric to it when I'm not 'there'? Would it be okay to interupt the common neutral? I've never seen a wall type switch for 240V.

[HayZee518]

cutting off the neutral or ground wouldn't have any effect. 240 is using the two outside legs anyway. get yourself a single 30 amp breaker panel and wire it between your source and the compressor. use the breaker as a switch.

[mrcaptainbob]

30 amp breaker....

I can do the breaker box. That's just as good. But....this is another fine example of my lack of understanding stuff. How can the 240 work if there's no neutral? 120 needs neutral to work, why not 240? What makes it so special? There were several light sockets in this old house that were wired (incorrectly) where the switch was on the cold side of the bulb. These are all corrected. Was thinking I could (incorrectly) control this compressor outlet by switching the neutral. Not understanding...

[HayZee518]

Let's say a dryer runs on 240 volts, ok? The heater elements operate on 240 - no neutral involved. The timer motor operates on 120 volt. The timer is using part of the line to the machine. The tumbler motor is rated at 240 volts not 120. Inside the motor there are two 120 volt windings connected together at their midpoint - if you will - a series connection. That is the two windings are additive as far as voltage is concerned. Oh by the way I forgot to mention, when you get that breaker panel, get one that is called a main lug panel and a double pole 30 breaker. Line side from your house or wherever the supply is goes to the lugs and neutral strip. The load for the compressor comes off the two breaker terminals and neutral strip. Two wire with ground is sufficient. you don't need a three wire with ground.

[mrcaptainbob]

240v.

Okay, so then a 240 unit (say a motor) does not need either a ground or a neutral? I can understand a water heater, as the 'power' is turned into heat and would, in effect, be zero at the end and would not need neutral. BUt does a 240 motor turn it all into 'energy' and also therefore nt need a neutral or ground?
Bytheway, I'll follow through on that breaker panel suggestion. THanks for the help, Hayzee.

[HayZee518]

yeah, you could compare the single phase compressor motor to that of a water heater - odd I didn't think of that the motor operates on the two hots no neutral involved. remember AC is alternating current. at any given moment in time, one hot is conducting and the other is at zero, then they switch, the other is conducting while its partner is at zero - so they're contually swapping positions. the two phases are 180 degrees out of phase with one another. That's why they "work!" Three phase is different in that there are three phases or windings, each 120 degrees out of phase with one another. Any motor or heater has to add up to 360 degrees, a circle if you will to operate properly. a sine wave which is what AC is goes from zero to 180 to 360 - a complete cycle.

[mrcaptainbob]

Phasing.....

UEREKA, Hayzee!! Then 120, being single phase, needs the neutral! Is that correct? The others don't....All the 'product' needs is a ground for protection.

[driz]

I think I got a 240 switch at Lowes

Would like to be able to turn off the remote air compressor. It will be switched over to 240V when it gets shoved up to the barn loft. Although it has it's own on/off cycle of coarse, but how can I shutoff the electric to it when I'm not 'there'? Would it be okay to interupt the common neutral? I've never seen a wall type switch for 240V.

Its been a few years but I rigged a similar setup to yours with my compressor in the basement. I just went and bought a simple wall switch rated for 240 Volts. Nothing special its mounted in a standard electric box and looks like any other light switch from the outside. I believe it cost 8 or 10 bucks.

[HayZee518]

True driz but if you read the information on the switch - it is TUNGSTEN rated at 277 volts - Lighting ONLY not motor rated. Motors create an inductive load to the switch which is very destructive to the contacts.

[driz]

Don't know whats on the back

It has worked well enough for my off and on use for the last 5-7 years. I will keep that in mind though if it decides to go tits up someday.

[mrcaptainbob]

Tungsten switch.

Hi, Hayzee. The 'induction' you mention messing with the contacts. Does that mess them up when the switch is in the on postion, and the electricity is just traveling through them? Or is it that they get messed up when using them to switch on/off while the motor is running?

[HayZee518]

when a motor starts it acts like a dead short across the line. it takes some current to get the rotor spinning with a load connected to it. that's why they have LRA [locked rotor current] listed on the motor. anyway, when it's spinning it's using the electricity to keep it running, but when you open the feed, it still spins and being within a magnetic field, induces a counter emf [electromotive force] to the line. in that split second(s) when the switch is opened, there's arcing at the contacts with that opposing emf that tends to gradually destroy the contacts. think of it as a collapsing magnetic field. lights don't exhibit that phenomenum because lights is a resistance and don't make use of a magnetic field.

[HayZee518]

Bob, I work part time for a local guy that owns a hydro-electric plant here in St Regis Falls. It's a small plant consisting of two generators. Combined it's less than a megawatt station at 4800 volts outgoing. On the high line going out there is a three pole knife type disconnect that's manually operated from the bottom of the pole [wooden] When the generators are not running, it's a simple matter of operating the disconnect and opening the line. There's a little arc drawn because of the "static" load, but it opens the line easily. Once we couldn't get the machines off line because the machine's circuit breakers were hung closed. And we had to disconnect off the line. We operated the disconnect only this time we drew a four foot arc between the high side at the pole and the three knife switches before eventually there was not enough of ionized air to continue the arc so the arc blew itself out. Actually the generators were in "motor" which means they were spinning as a motor not a generator. It's this counter emf that made the arc at the pole.

[driz]

Heres The Best Way To Get This Sort Of Stuff

Instead of looking all around and wondering just go to the electric supply store with a drawing of what you will be doing. Make sure you have the motor size or model of the compressor you will be using. It might cost you a couple bucks more but its made up in convenience. They will set you up with the stuff you need to make code, do it right and likely will know those little considerations to make the job better you tend to forget if you aren't in the business. That way you get everything you need and there is no worries. I do almost all my own everything but electricity is one of those things that the counter guy can be a big help with. This is one of those places where Home Depot isn't usually the best source for your stuff. A knowledgeable counter guy can make it go so easy..............................